Cadmium Tungstocyanide, Cd₂[W(CN)₈]

Cadmium Tungstocyanide, Cd₂[W(CN)₈].8H₂O, may be prepared from the silver salt by double decomposition, or by the following method due to Rosenheim and Dehn. A solution of 12-tungstosilicic acid in concentrated hydrochloric acid is reduced electrolytically until the solution becomes greenish yellow in colour. Strong alkali is added to remove the excess of acid, and solid potassium cyanide is then added in considerable quantity. On evaporation of this solution, potassium chloride separates. A similar solution may be obtained more directly by treating the compound K₃W₂Cl₉ with potassium cyanide. The solution is now treated with a concentrated solution of cadmium sulphate and digested on the water-bath; an impure cadmium salt separates which is dissolved in concentrated ammonia solution, and the solution, placed in a freezing mixture, is saturated with ammonia. Cadmiumtriammine tungstocyanide, [Cd(NH₃)₃]₂[W(CN)₈].2H₂O, separates in long golden-yellow needles, which on washing with water, or on exposure to air, lose ammonia and yield the corresponding diammine salt, [Cd(NH₃)₂]₂[W(CN)₈].2H₂O. On treatment with 1 to 2 per cent, sulphuric acid, microscopic yellow prisms of cadmium tungstocyanide, Cd₂[W(CN)₈].8H₂O, are obtained. When exposed to sunlight the crystals turn red but no change in weight occurs, and on heating, the yellow colour returns. When heated to 125° C. 7 molecules of water are lost. The salt is only slightly soluble in water, but is more readily soluble in hydrochloric acid.

The cadmium salt may also be obtained from the pyridine compound W(OH)₂(SCN)₃.2C₅H₅N. which dissolves in potassium cyanide solution, forming a blue solution which turns yellow on warming and can be made to yield the above-mentioned triammine salt.